This invention relates to disposable medical blood pressure measuring devices that are utilized with in vivo catheters to measure and record the blood pressure of a patient. Basically in vivo blood pressure monitoring is widely used in hospitals and includes a liquid filled catheter system that is introduced into the particular blood vessel to be monitored. The open end of the liquid filled catheter thus is placed within the patient's blood vessel and a continuous column of liquid takes up the space between that catheter end and a pressure transducer. The blood pressure may therefore be directly measured by the transducer remote from the patient by measuring the fluctuations of pressure at the external end of the liquid column.
Typically such pressure transducers comprise strain gauges in the form of cantilever beams for example in U.S. Pat. Nos. 4,545,389 and 4,683,894, or may comprise tiny silicon chips with a movable diaphragm etched into the chip. The diaphragm divides the pressure transducer into two chambers, one of which is referred to as the patient side chamber and contains the liquid within the liquid column to the patient. The other chamber is vented to atmosphere such that pressure readings taken of blood pressure from the patient side of the diaphragm are absolute values.
The diaphragm and thus the pressure transducer therefore flexes to provide readings of blood pressure based upon the differential pressure between the patient chamber and the vented chamber. The transducer then electronically senses the amount of flexing of the diaphragm or chip and translates the amount of flex into an electrical signal that is provided to a electronic monitor that reads out the patient's blood pressure for the attending personnel.
Because the reading is based upon the flexing of a diaphragm or other member separating the patient side chamber from the vent side chamber, it is important that the diaphragm have a zero reading when the two pressures are equal, that is, when the differential pressure between the chambers is zero. Due to manufacturing constraints, and due to the relatively low cost of such disposable pressure transducers, such pressure transducers are manufactured with a zero offset, that is, when the differential pressure is zero, there is some flexure to the diaphragm and therefore the electronic monitor would not read zero pressure for pressure transducers without some compensation.
Accordingly, before use, it is necessary to determine the particular zero offset for each pressure transducer and adjust the electronic monitor to take that offset into account and to insure that the monitor gives a true reading of zero when both chambers within the transducer opposing the diaphragm are at equal pressures.
Typically, the zero offset is determined by venting the patient side chamber to atmospheric pressure. When it is so vented, therefore, both chambers on either side of the diaphragm are at the same pressure, atmospheric, and any reading then present based on flexure of the diaphragm is the zero offset.
As a convenient means of venting the patient side or chamber to atmospheric pressure, the pressure transducer is normally supplied with a two-position stopcock where, in one position, the patient chamber is connected to the patient catheter and in the other position, the patient is cut out and the patient side chamber is vented to atmosphere through a side port of the stopcock having a standard female Luer fitting.
The venting side port of the stopcock is fitted with a deadender cap and the cap is removed during the venting procedure and reapplied after the zero offset has been determined. One of the problems in such operation, however is that the deadender cap is quite small and is easily dropped during or after its removal while the operator is taking the zero reading and whose attention therefore is diverted elsewhere.
Once dropped the cap is no longer sterile and the user must locate and affix another sterile deadender cap to the vent port in order to further use the pressure transducer with a patient.
Alternatively, the open end of the female Luer fitting of the vent port in the stopcock is uncovered during the zeroing procedure and thus can be touched by a non-sterile object and its sterility breached. In either event, the procedure is delayed and is inconvenient for the user.
In transit, a dust cap is generally fitted over the side port vent Luer fitting and thus an extra part is needed in the sterile package since in use, the dust cap is removed and the deadender, supplied in the same package applied.